An optical disk possesses great advantages in that the recording medium, in the form of a disk, can be removed from the recording/reproduction device, and the recording medium is inexpensive. Therefore, optical disk devices preferably require the advantages of high speed and high recording density without losing the features of disk removability and low cost.
To record information on a magnetic disk or magneto optical disk (MO), the magnetic head is brought near the recording medium. However, during high density recording, the magnetic head is preferably separated from the recording medium as much as possible to achieve high speed.
The principle of irradiating light onto a recording layer to record information is well known. The process has the advantage that an information recording medium is obtained which is capable of being rewritten many times, utilizing a phase change (also called phase transition) in the material that changes the arrangement of the atoms using heat. In the method disclosed in JP-A No. 344807/2001, for example, the basic structure of such a phase-change optical disk is composed of a protective layer formed over a substrate, a recording layer (or film), such as a GeSbTe alloy, another protective layer, and a reflective layer. A multilayer phase-change optical disk of up to four layers is reported to be currently under development.
On the other hand, optical disks utilizing organic material, which are used as a CD-R and DVD-R disk, have been produced for practical commercial use. These disks comprise a recording layer containing pigments that absorb the light wavelengths of the recording light source. Recording is performed on these disks by irradiating a laser light to induce a change in the material on the substrate surface in contact with that recording layer.
Besides a magnetic disk formed of a thin magnetic film, a method was also disclosed utilizing fine magnetic particles, such as FePt, in [Patent document 1] JP-A No. 344807/2001. The medium based on this method preferably has multiple layers for increasing the effective recording density (effective surface density). However, at three or more layers, the transmittance ratio and the recording sensitivity have a tradeoff relationship (one improves while the other becomes worse), so that one of either the reproduction signal quality or the recording sensitivity is sacrificed (becomes worse) at the expense of the other.